A predictive formula for the H-mode electron separatrix density: Bridging regression and physics-based models across C-Mod, AUG and JET tokamaks
Abstract
The electron density at the separatrix () plays a central role in balancing energy confinement, detachment achievement, and ELM suppression in tokamaks, thereby influencing core-edge integration. To study what determines this key parameter, a database of H-mode separatrix density measurements from Alcator C-Mod, ASDEX Upgrade, and JET tokamaks has been assembled using a consistent analysis method across all devices. This dataset is used to derive a regression scaling expression based solely on engineering parameters, and the results are compared to predictions from the two-point model. The agreement found is remarkable: both the regression and model provide similar parameter dependencies and tokamak-specific multiplicative constants. Building on this agreement, a fully predictive formula that combines the regression dependencies and the two-point model multiplicative constant is proposed. This formula is able to estimate across the three machines within a factor of 1.5, and provides projections to next-step devices (ITER, SPARC, DTT, JT-60SA and COMPASS-U) that are in agreement with available SOLPS simulations.
Cite
@article{arxiv.2601.23140,
title = {A predictive formula for the H-mode electron separatrix density: Bridging regression and physics-based models across C-Mod, AUG and JET tokamaks},
author = {D. Silvagni and O. Grover and A. Stagni and J. W. Hughes and M. A. Miller and B. Lomanowski and L. Balbinot and G. Ciraolo and W. Dekeyser and M. Dunne and L. Frassinetti and C. Giroud and T. Happel and I. Jepu and A. Kallenbach and A. Kirjasuo and A. Kuang and T. Luda and D. Moulton and O. Pan and C. Perez von Thun and T. Puetterich and G. Rubino and S. A. Silburn and H. J. Sun and D. Umezaki and H. Zohm and the ASDEX Upgrade team and JET contributors and the EUROfusion tokamak exploitation team},
journal= {arXiv preprint arXiv:2601.23140},
year = {2026}
}